What organ do fish use to extract oxygen from water?

Fish use gills to extract oxygen from water. As water flows across the gill surfaces, dissolved oxygen moves into the bloodstream while carbon dioxide moves out.

Gills contain thin filaments and smaller lamellae that provide a large, moist exchange surface. Blood and water flow in opposite directions across these surfaces, a countercurrent arrangement that helps fish remove oxygen efficiently even when its concentration is low.

Different fish ventilate their gills in different ways. Many pump water by opening and closing the mouth and gill covers, while sharks and some other species rely heavily on forward swimming or specialized pumping. Gills are not the same as lungs: lungs are adapted primarily for extracting oxygen from air, whereas gills are delicate structures designed for water.

Scales protect the body, fins provide steering and propulsion, and the lateral line detects movement. None of those structures performs the main gas-exchange job.